Fuel injected engines employ fuel injectors, each of which delivers a metered quantity of fuel to an associated engine cylinder during each engine cycle. Prior fuel injectors were of the mechanically or hydraulically actuated type with either mechanical or hydraulic control of fuel delivery. More recently, electronically controlled fuel injectors have been developed. In the case of an electronic unit injector, fuel is supplied to the injector by a transfer pump. The injector includes a plunger which is movable by a cam-driven rocker arm to compress the fuel delivered by the transfer pump to a high pressure. An electrically operated mechanism either carried outside the injector body or disposed within the injector proper is then actuated to cause the fuel delivery to the associated engine cylinder.
The injector may include a valving mechanism comprising a spring-loaded spill valve and a spring-loaded direct operated check (DOC) valve wherein the former is operated to circulate fuel through the injector for cooling, to control injection pressure and to reduce the back pressure exerted by the injector plunger on the cam following injection. However, the need to separately control two valves leads to the requirement for two separate solenoids to control the valves. Besides adding to the overall cost of the injector, the need for two solenoids undesirably increases component count and undesirably increases the overall size of the injector and/or decreases the space available inside the injector for other components.
The electromagnetic force exerted by a solenoid coil increases as the air gap length of the solenoid is reduced. Variability in the air gap length due to assembly tolerances causes a force variability from solenoid-to-solenoid even if current is carefully controlled. This variability can be accommodated in fuel injectors of the foregoing type by selecting spill valve and DOC valve springs and coil current magnitudes which are large enough to work for all cases. However, this method undesirably leads to higher spring loads and electrical currents then would otherwise be needed if no variability existed in the solenoid characteristics.